Factor them both.
1 x 5 = 5
1 x 7 = 7
Discard the common factor (1) and combine the remaining ones
5 x 7 = 35
35 is the lowest (least) number that both 5 and 7 will go into evenly.
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The LCM of 5 and 35 is 35.
5
The LCM of 5, 19, and 35 is: 665
7 is prime, so it's lowest common multiple with anything that isn't itself a multiple of 7 (in which case the LCM would just be the other number itself) is that number times seven (in this case, 175).
140